The present invention relates to a method of determining correction parameters for use in adjusting the amounts of light to which originals are to be exposed in a color copying operation in dependence on the composition deviations of the respective originals from a calibrating original. More particularly, the present invention is related to a correction-parameter determining method in which the significance of associated regions of the original which are essentially of the same color, or so-called color dominants, is suppressed during the determination of the correction parameters.
It is well known that, during the production of photographic positives from color negative originals, a copying arrangement copies the color negative onto the positive material using the three basic colors red, green and blue. In order to obtain an optimum positive, it is necessary to use precise amounts of light in these colors during the copying of the negative on the photosensitive positive material, which amounts, in most instances, vary from one negative or original to another. In view of the high copying speeds of currently used copying apparatus or printers, these correct light amounts in the various colors cannot be selected by an observer or copying machine operator; rather, these light amounts must be determined in accordance with objective criteria.
In the following discussion, it will be assumed, for the sake of simplicity but without limiting the general application of the method of the present invention, that the copying apparatus is of the type wherein the changes in the copying light amounts in the three colors are achieved by varying the corresponding times of exposure. A copying apparatus capable of performing in this manner is disclosed, for instance, in a commonly owned U.S. Pat. No. 4,101,217 granted on July 18, 1978 to Fergg el al, to which reference may be had for details of the copying apparatus, if necessary. The copying apparatus is so adjusted ("basic adjustment") that a well-exposed negative image which is average as to its color and light distribution ("standard original") results in an optimum positive image at equally long times of illumination of the original in the three colors. Now, in order to obtain the best positive image from an arbitrary negative image which is structured differently from the standard original, certain corrections must be made in the interval of exposure in the various colors. Such corrections involve, on the one hand, the average exposure ("density correction") and, on the other hand, the illumination ratios in the various colors ("color correction"). The present application is concerned with the objective determination of the color corrections.
There have already been made numerous proposals for solving this problem. Many commercially available printers or copying apparatus operate on the following basis: with respect to most images, the color balance (that is, the relative proportions of the basic colors) is not far removed from the balance which is obtained by averaging over a multitude of images. Now, if the color balance of a particular original or negative, which is obtained by measuring the latter, deviates from the equilibrium of the standard original, then corrections are made in the copying light amounts or intervals relative to the basic adjustment of the printer or copying apparatus. Of course, when this approach is resorted to, it results in incorrect positive images when the original in question bears an image whose balance was far removed from the average value ("color dominant"). The current methods based on the aforediscussed principle scan the negative or original at a multitude of spatially distributed regions, and evaluate the measured values obtained during the scanning in accordance with certain criteria. Under these circumstances, it is often necessary to preliminarily evaluate a great number of negatives or originals before it is possible to draw proper conclusions from the scanned individual values as to the needed color correction. This, of course, is very disadvantageous.